Expression of the recently discovered human restriction factor SAMHD1 is responsible for the infection block imposed to lentiviruses such as HIV-1, HIV-2 and SIVmac by primary macrophages, dendritic cells and resting CD4+ T-cells. SAMHD1 blocks lentiviral infection by preventing the occurrence of reverse transcription. The viral accessory protein Vpx, contained in SIVmac and HIV-2 particles, overcomes the SAMHD1 reverse transcription block by inducing SAMHD1 degradation. SAMHD1 is a dGTP-regulated deoxynucleotide triphosphohydrolase that decreases the cellular levels of triphosphodeoxynucleotides (dNTPs). Interestingly, cycling and non-cycling cells express SAMHD1; however, SAMHD1's antiviral activity is only observed in non- cycling cells. Our preliminary findings correlate the lentiviral restriction phenotype observed in non-cycling cells with the phosphorylation and S-glutathionylation state of SAMHD1. This proposal will test the hypothesis that SAMHD1 antiviral activity is regulated by phosphorylation and S-glutathionylation. The following specific aims will be used to address this hypothesis. Aim1 will explore the role of SAMHD1 phosphorylation in retroviral restriction. Aim 2 will explore the role of S-glutathionylation in retroviral restriction. Aim 3 will explore the role of SAMHD1 in the type I IFN response. Overall, this proposal will establish the regulation of SAMHD1 antiviral activity. Understanding the regulation of SAMHD1 is instrumental for the development of novel anti-HIV-1 vaccine strategies since overcoming SAMHD1 increases the adaptive immune response during infection of dendritic cells and macrophages.